(1) Field of the Invention
This invention relates generally to the field of electrochromic display devices and, more particularly, to electro-optical devices and methods utilizing working and counter electrodes on opposite sides of a substrate for single and double sided electrochromic displays.
(2) Description of the Related Art
An electrochromic display utilizes an electrolyte layer in contact with an electrochromic material. By applying an electrical potential across an interface between the electrolyte and the electrochromic material (or, in the case of a soluble electrochromic material, between the electrode and the electrolyte containing the dissolved electrochromic material), an electrochromic effect can be generated which changes the color or optical density of the electrochromic material. Examples of these electrochromic displays can be found in International Patent Application No. PCT/US93/12431, International Publication No. WO 94/15246, filed Dec. 21, 1993, the disclosure of which is incorporated herein by reference.
Electrochromic materials can be dispersed in an electrically conductive, substantially ionically isolative layer which contacts an electrolyte layer. Electrodes can be provided for applying a voltage across an interface of the ionically isolative layer and the electrolyte layer in order to generate an electrochromic effect at the interface. In one type of electrochromic device, the electrolyte and ionically isolative layers are sandwiched between a vertically stacked pair of a working electrode and a counter electrode. Upon applying a voltage across the electrodes, an induced current flows in a single direction across the interface of these layers to generate an electrochromic effect having a single color. Thus, a single-sided electrochromic display is produced that can display an image in a single color. However, because the electrochromic color change takes place at the interface between the two electrodes, one electrode must be transparent. Transparent electrodes typically have a high resistance and, therefore, can significantly decrease the overall conductivity of the display device and increase the voltage required for switching. Another drawback of transparent electrodes is that they are expensive compared to conventional printed circuitry fabricated with conventional silver ink, at least insofar as the finished product is concerned.
Another example set forth in the above-referenced international application includes the use of side-by-side counter and working electrodes having a common ionically isolative layer and a common electrolyte layer laminated thereon. Because the electrochromic effect generated at the interface of these layers is positioned above the counter and working electrodes, neither electrode need be transparent. However, when a voltage is applied to the working electrode, an induced current will flow from the working electrode and across its interface region in a first direction, through the electrolyte layer, and then across the interface region above the counter electrode in an opposite direction. As a result of the current crossing the two interfacial regions in opposite directions, the electrochromic effect generated above each electrode will have a different color from the other due to opposite changes in the oxidation state of the electrochromic material.
Thus, for a given electrochromic material, this display may be incapable of displaying an image with only a single color unless extensive masking is provided. If the electrochromic material must be chosen such that an electrochromic effect is only generated upon the gain or loss of an electron, such a selection must be made from a more limited group of suitable materials. Moreover, to utilize this electrochromic display to generate a particular image, the working and counter electrodes must be arranged in particular image-defining patterns. Because both of these electrodes are positioned on the same side of a substrate material, the physical layout for these electrodes can become unduly complicated, particularly where a relatively elaborate image, such as one containing alphanumeric characters, is to be displayed. Additionally, because side-by-side electrodes are used on the same side of the substrate, twice as much substrate area is needed than if only one electrode were utilized on each side.
U.S. Pat. No. 5,446,577 to Bennett et al. is directed to display devices having a transparent outer layer, a first electrode having a reflective surface facing the transparent layer, an electrochromic material located between the reflective surface and the outer transparent layer, an electrolyte in contact with the electrochromic material and a second electrode located behind the first electrode. The first electrode is ion-permeable, allowing ions to pass through and contact the electrochromic material in order to alter the optical properties of the material. This reference states that a two-sided display structure is possible, but indicates that such a device would include a third electrode functioning as a counter electrode in the interior of the device, and that the visible electrodes would both be working electrodes. There is no suggestion of a two-electrode, two-sided structure in which the working and the counter electrode both provide electrochromic color changes, nor is there any suggestion of effecting an electrochromic color change by the reversible, electrical deposition of a metal dissolved (as a metal salt) in an electrolyte.
It would, therefore, be desired to have a double-sided electrochromic display device which can be configured to display a particular image in a single color, where desired, without an excessively complicated layout pattern for the working and counter electrodes. Additionally, a double-sided electrochromic display device is also needed where separate images can be displayed on each side of the device, and which requires a minimum of masking and substrate area.